1. Field of the Invention
The present invention relates to a lithium ion secondary battery and, more particularly, a lithium ion secondary battery which exhibits excellent battery characteristics by improving the constitution of a negative electrode.
2. Description of the Related Art
Recently, nonaqueous electrolyte batteries using lithium as their negative electrode active materials have attracted attention as high-energy-density batteries. As an example, primary batteries using, e.g., manganese dioxide (MnO.sub.2), carbon fluoride [(CF.sub.2).sub.n ], and thionyl chloride (SOCl.sub.2), as their positive electrode materials are widely used as power sources of calculators and watches and backup batteries of memories.
In addition, with recent reduction in size and weight of various electronic devices, such as VTRs and communication devices, a demand has increasingly arisen for high-energy-density secondary batteries as power sources of these devices. To meet this demand, active researches have been made on lithium secondary batteries using lithium as a negative electrode active material.
These researches have been made on a lithium secondary battery comprising a negative electrode consisting of lithium; a nonaqueous electrolyte, in which a lithium salt, such as LiClO.sub.4, LiBF.sub.4, or LiAsF.sub.6, is dissolved in a nonaqueous solvent, such as propylene carbonate (PC), 1,2-dimethoxyethane (DME), .gamma.-butyrolactone (.gamma.-BL), or tetrahydrofuran (THF), or a lithium-ion-conductive solid electrolytic salt; and a positive electrode containing an active material mainly consisting of a compound which topochemically reacts with lithium, such as TiS.sub.2, MoS.sub.2, V.sub.2 O.sub.5, V.sub.6 O.sub.13, and MnO.sub.2.
No lithium secondary battery with the above arrangement, however, has been put into practical use. This is so mainly because the charge-discharge efficiency is low and the number of times by which charge and discharge are possible is small (i.e., the cycle life is short). It is considered that the major cause for this is degradation in lithium due to the reaction between lithium of the negative electrode and the nonaqueous electrolyte. That is, the surface of lithium which is dissolved as lithium ions in the nonaqueous electrolyte during discharge is partially inactivated by reacting with the nonaqueous solvent contained in the electrolyte when it precipitates from the nonaqueous electrolyte during charge. As a result, when charge and discharge are repeatedly performed, lithium precipitates into dendrites or globules or is desorbed from a collector of the negative electrode.
For these reasons, lithium ion secondary batteries having negative electrodes containing carbonaceous materials which absorb and desorb lithium ions, such as coke, a resin sintered product, a carbon fiber, and pyrolyric carbon, have been proposed. A lithium ion secondary battery having a negative electrode of this type can reduce degradation in negative electrode characteristics by suppressing the reaction between lithium and the nonaqueous electrolyte and hence the precipitation of dendrites.
It is considered that in the negative electrode containing the above carbonaceous material, absorption and desorption of lithium ions occur in a portion of a structure (graphite structure) in which hexagonal-net-plane layers consisting primarily of carbon atoms are stacked, particularly in portions between these hexagonal-net-plane layers, thereby causing charge and discharge. It is, therefore, required to use a carbonaceous material in which a graphite structure is developed to some extent, as the negative electrode of a lithium ion secondary battery. However, when the carbonaceous material obtained by powdering large size crystals which are highly graphitized is used as a negative electrode in the nonaqueous electrolyte, Li ions are absorbed to the graphite crystal in only one direction and are desorbed from the crystal in the opposite direction. For this reason, a current concentrates on the side portions of the graphite crystals. As a result, the nonaqueous electrolyte is decomposed to decrease the capacity and the charge-discharge efficiency of a battery. In addition, an overvoltage increases in a rapid charge-discharge cycle, so that precipitation of a lithium metal poses a serious problem. Therefore, when a lithium ion secondary battery having the above negative electrode is operated at a high current density, the capacity, charge-discharge efficiency, and the voltage during charge-discharge of the battery decrease significantly. As the charge-discharge cycle progresses, a decrease in capacity becomes large to undesirably shorten the cycle life.
Furthermore, like the negative electrode containing the carbonaceous material obtained by powdering giant crystals, a negative electrode containing a fine powder of carbon fibers which are highly graphitized decomposes the nonaqueous electrolyte, with the result that the performance as the negative electrode is largely degraded.
Jpn. Pat. Appln. KOKAI Publication Nos. 62-268058, 2-82466, 4-61747, 4-115458, 4-184862, and 4-190557 propose control of the degree of graphitization and optimal parameters for graphite structures of various carbonized and graphitized products. However, negative electrodes containing these materials do not necessarily have sufficiently good characteristics. Jpn. Pat. Appln. KOKAI Publication Nos. 4-79170, 4-82172, and 5-325967 disclose carbon fibers used as negative electrodes. These negative electrodes using carbonaceous materials obtained by powdering these carbon fibers have problems on large-current discharge characteristics and the capacity density (mAh/cm.sup.3) of the negative electrodes.